Fix crash when using [partial] filtering on rebased review

When rendering the "Squashed History" log, rebase information can
be used only if all commits in the review are included.  This is
always the case for the [reviewable], [relevant] and [full]
filters.  In the code, [full] and [partial] are both called
"files", with the difference being that [partial] also includes
URI parameters selecting a range of commits.  When deciding whether
to include rebase information when rendering the "Squashed History"
log, however, "files" was (incorrectly) assumed to be [full],
which caused a crash (assertion failure) if the filter was actually
[partial].
Fix handling of rebases in "Squashed history" log display

Include the rebase ids instead of passing None.  (I don't recall
why None was used here when rebase ids were introduced; the ids are
as readily available here as anywhere else.)

Also drop unnecessary filtering of prepared-but-not-performed
rebases; the database query already selects performed rebases.
(And the filtering was broken either way; it looked at old_head
instead of new_head, and neither can be None at that point.)
Make /showcommit's 'file' parameter support paths in addition to ids

This makes it easier to manually construct meaningful URLs (I have missed
this feature at times) for instance for use in test cases.
Extend 020-reviewrebase.py to check /showcommit's log as well

When /showcommit is loaded wia one of the filters [reviewable], [relevant]
and [full], the "Squashed History" log will be identical to the one on the
review's front-page, including information about rebases, so load it too
and apply the same check as to the on the review front-page's log.

This acts as a regression test for the "Fix handling of rebases in
"Squashed history" log display" commit in r/207.  (Not a perfect one; the
log is sometimes (randomly) rendered correctly even without the fix.)
Extend 020-reviewrebase.py to test the [partial] filter a little

This tests that the "Fix crash when using [partial] filtering on rebased
review" commit in r/207 does its job correctly.